The invention relates generally to hybrid propulsion systems, and in particular to a system and method for distributed energy storage in heavy duty hybrid propulsion applications.
Some vehicles use electric traction motors to propel the vehicle. Typically, the electric traction motors are connected to a link, such as a bus, that provides the motors with power. One or more on-board alternators may be used to provide the power to the link. In certain operating conditions, such as when the vehicle is decelerating or is maintaining speed on a downhill grade, the back-emf produced by the electric motors is greater than the voltage provided by the engine-driven alternator. Under such conditions, the electric traction motors cease acting as motors and become alternators. This process, known as dynamic braking, is a form of electric braking that is used to reduce wear on the mechanical brake system components on a vehicle. In the case where the vehicle is a locomotive, dynamic braking reduces brake wear on the locomotive and also all of the rail cars within the train, Typically, a resistor is used to dissipate the electric power as heat produced by the electric motor during dynamic braking.
Hybrid propulsion systems have been developed to recover some of the energy that is wasted as heat during dynamic braking. The recovery of this wasted energy is known as regenerative braking. Vehicles having heavy duty hybrid propulsion systems, such as buses, large trucks, mining vehicles, and locomotives, may require massive energy storage units, generally comprised of batteries, ultracapacitors, flywheel, or combinations of one more of these technologies. One example is a heavy haul hybrid locomotive propulsion system. In such applications, motive power is generally provided by a prime mover, such as a diesel engine, which is directly coupled to an alternator and an associated high power rectifier that converts the output of the alternator from an alternating current (AC) to a direct current (DC). The output of the rectifier is then coupled to a main DC link that may supply several motors with power. Typically, the energy storage unit is electrically connected to the main DC link through an electronic DC/DC converter that is controlled by an energy management system and associated vehicle system controls. The DC/DC converter provides a bi-directional DC-DC interface for the energy storage unit so that the energy storage unit is operable to supply power to the traction motors and to receive power from the traction motors during regenerative braking. The power from the traction motors is used to partially re-charge the energy storage unit. Thus, the energy that is normally dissipated in a grid resistor as heat during dynamic braking is recovered and used to partially re-charge the energy storage units. Later, the energy storage unit can be discharged to supply power to the traction motors. With proper system controls, the hybrid propulsion system can be used to provide vehicle acceleration with a reduced output power from the diesel engine, thus reducing the amount of fuel required for a given mission as compared to a conventional non-hybrid locomotive.
However, there are a number of problems associated with existing hybrid propulsion systems. For heavy duty vehicles, a bi-directional DC-DC energy storage interface typically is required due to the smaller power rating of the energy storage units compared to the power rating of the prime mover. However, the electronics required for the bi-directional DC/DC converter significantly increase the cost of the power electronics hardware. For example, during normal operation of a heavy duty vehicle, the voltage of the main DC link typically varies from approximately 250 V to 1,500 V. To reduce the cost of the DC/DC converter, the energy storage unit's output voltage is typically chosen to be either above or below the DC link during hybrid mode of operation. However, when the energy storage system in the hybrid propulsion system is required to operate both above and below the DC link voltage, an “H” bridge configuration is generally used in the DC/DC converter. However, the “H” bridge configuration requires at least twice the number of power electronic switches, significantly increasing cost of the DC/DC converter. A DC/DC converter of a high power rating also presents a potential reliability issue. Furthermore, due to the higher power requirements, the energy storage units of conventional hybrid propulsion systems require parallel operation of multiple smaller energy storage units. However, power sharing within the parallel energy storage units may be a problem during operation over a wide range of environmental temperature extremes. If power sharing is not adequately controlled, the life of the energy storage units may be reduced.
Still further, in conventional heavy duty hybrid vehicles, various electrical devices (such as electrical lights, fans, air compressor) are supplied power from the engine. This means that the engine is required to operate even when the vehicle is braking or running on a downhill grade to supply power to the electrical devices. This results in significantly reduced fuel economy.
Accordingly, techniques that decrease the cost and/or improve the energy efficiency of hybrid propulsion systems are desirable. More specifically, techniques that increase the amount of regenerative braking power produced by the hybrid propulsion systems of heavy duty vehicles that may be recovered are desirable.